Knowledge of electrical resistivity of glass in a production melter or channel would be very useful. In an electric melter the resistivity controls power dissipation. In either a gas or electric melter the monitoring of any quality relating to glass composition, and thus to viscosity, could signal possible upset conditions in time to make compensating adjustment in the subsequent forming operations.
The sophistication and sensitivity of most laboratory apparatus prohibit application in the hostile field environment. To minimize the effect of temperature variations, any physical property measurement in a melter should be confined to a small region. The device of this invention measures both resistivity and temperature near the center of a very small sample of glass.
The resistance is the ratio of voltage, or potential drop, to current across some part of a circuit. In an electrolyte such as molten glass, the distribution of potential can be controlled by the design of the electrodes. Of particular significance is the fact that an extreme disparity in sizes of the electrodes serves to concentrate the major portion of the potential very close to the smaller electrode. In the limiting case, the nature of the larger electrode and the separation distance become irrelevant, the net resistance depending only on the geometry of the small electrode and the resistivity of the electrolyte. A small, 2 mm to 10 mm diameter, sensor electrode and a much larger return electrode which has a surface area of at least 100 times that of the sensor electrode are used to complete a high frequency circuit through the molten glass. Thus, this single small sensor electrode can be calibrated in a laboratory solution of known resistivity, and then used to measure the resistivity of another electrolyte. The measurement of the resistivity is independent of the amount of fluid or size or geometry of either container or the size and shape of the return electrode.
The sensor electrode consists of a type R (platinum versus platinum--13 percent rhodium) thermocouple junction in contact with the bottom of a double bore alumina tube. The sensor electrode is constructed of heavy wire to minimize resistance. The small size and structural simplicity of the electrode make insertion into a furnace or glass melt relatively easy. There are no mechanical parts in the electrodes to be affected by the molten glass or high temperature. The sampled region in the resistivity measurement is nearly a sphere a few centimeters in diameter centered at the sensor electrode. A high frequency is used to eliminate polarization and interference from other electrical sources. By using the sensor electrode as a thermocouple, temperature is measured at the common location the resistivity was measured at.